One type of article surveillance system includes an inductive magnetic field generator for deriving a first magnetic field having a predetermined frequency. An article to be monitored includes a structure responsive to the first magnetic field for deriving a second magnetic field having a predetermined frequency. A receiver for the predetermined frequency of the second inductive magnetic field provides an indication of the presence of an article in a monitored region between coils of the generator and receiver by activating an alarm in response to the predetermined frequency of the second magnetic field being received for at least a predetermined interval.
Several different arrangements of the receiver coils have been employed. One of the most common types of receiver coils is a simple, single wire loop having a predetermined number of turns. The size of the loop is such as to cover a specific area or zone. The single wire loop arrangement has several disadvantages, one of which is that the size of the loop must be relatively large to cover a typical region to be monitored, such as a retail establishment exit. A large single wire loop is likely to be subjected to a high level of background magnetic noise. In addition, a large area wire loop has relatively low magnetic field sensitivity and is very orientation dependent. It is intolerable in virtually all article surveillance systems utilizing AC magnetic fields for the loop to be magnetic field orientation sensitive because of the completely random nature of the orientation of the emitting structure on the surveilled article relative to the magnetic field receiver.
To improve the performance of the large single loop coils, many article surveillance systems have employed coils shaped as a figure 8. A figure 8 coil includes two loops, with the wire forming the loops typically wound in opposite directions. An advantage of a figure 8 coil arrangement is that background noise incident on both loops is cancelled by the opposing directions of the windings or conductors forming each loop. In addition, the opposite winding directions of the figure 8 coils and the smaller size of the loops forming the figure 8 enable the figure 8 coil to be less orientation sensitive than a single loop.
It has been found, however, that the figure 8 coil arrangements are relatively insensitive to magnetic fields in the region of an intersection of the loops. Magnetic fields from the surveilled article incident on the coil arrangement in the vicinity of the intersection of the opposing loops have a tendency to be cancelled, to create a dead zone that is unresponsive to the magnetic field derived from the surveilled article.
It is possible to obviate the dead zone of the oppositely wound figure 8 loops by winding both loops in the same direction. However, the background noise level with such an arrangement is increased relative to the background noise which is induced in the oppositely wound figure 8 loops. Typically, signals derived from figure 8 loops wound in the same direction or in opposite directions have been analyzed by connecting the wires forming the two loops in series. Thus, a single signal is coupled from the loops to processing circuitry of the receiver.
It is, accordingly, an object of the present invention to provide a new and improved receiver coil arrangement for an inductive magnetic field surveillance system.
Another object of the invention is to provide an inductive magnetic field surveillance system with an improved receiver coil arrangement having relatively high sensitivity, immunity to background noise and without dead bands or orientation sensitivity.